And far away, as Frodo put on the Ring and claimed it for his own, even in Sammath Naur at the very heart of his realm, the Power in Barad-dur was shaken, and the Tower trembled from its foundations to its proud and bitter crown. The Dark Lord was suddenly aware of him, and his Eye piercing all shadows looked across the plain to the door that he had made, and the magnitude of his own foll was revealed to him in a blinding flash, and all the devices of his enemies were at last laid bare. Then his wrath blazed in consuming flame, but his fear rose like a vast black smoke to choke him. For he knew his deadly peril and the thread on which his doom now hung.
From all his policies and webs of fear and treachery, from all his stratagems and wars his mind shook free; and throughout his realm a tremor ran, his slaved quailed, and his armies halted, and his captains suddenly steerless, bereft of will, wavered and despaired. For they were forgotten. The whole mind and purpose of the Power that weiled them was now bent with overwhelming force upon the Mountain. At his summons, wheeling with a rending cry, in a last desperate race there flew, faster than the winds, the Nazgul, the Ringwraiths, and with a storm of wings they hurtled southwards to Mount Doom.
He did respond -he sent the Nazgul. But he was too late, the Ring was eaten, and he was defeated.